In packaging steps for printed circuit boards or electronic parts or components, electroless nickel/displacement gold plating has been widely used for years in surface treatments for applications where high reliability is required.
As displacement gold plating causes gold to deposit by making use of a difference in oxidation reduction potential between nickel as a substrate and a plating bath, gold penetrates into the nickel so that points of corrosion occur by oxidation (leaching). These points of corrosion by the oxidation may act as an inhibitory factor upon connecting Sn in a solder layer with Ni in the course of subsequent solder reflow, leading to a problem that the joint properties such as strength may be lowered.
When electroless nickel/displacement gold plating film is used at a site to be subjected to wire bonding, heat treatment is conducted after the plating treatment. By this heat treatment, however, Ni is caused to diffuse over the surface of the Au film so that the percentage of success in wire bonding is reduced. This reduction in the percentage of success in wire bonding is, therefore, prevented by further applying an electroless Au plating onto the electroless nickel/displacement metal film to increase the thickness of the Au film such that the diffusion of Ni over the Au film can be avoided. This countermeasure, however, involves a cost problem.
Keeping in step with the promotion of lead-free movement in recent years, on the other hand, there is a trend toward using new alloys composed principally of tin (for example, Sn—Ag and Sn—Ag—Cu) for joining without using tin-lead alloys. This trend is, however accompanied by a problem in that compared with conventional tin-lead eutectic solder, such new alloys result in the application of greater heat load during solder joining and hence in reduced joint properties.
It has, therefore, become a practice in recent years to avoid the above-described problem with a palladium film interposed between an electroless nickel plating layer and a displacement gold plating layer by electroless palladium plating. As reducing agents in electroless palladium plating baths for use in the above-described practice, phosphinic acid compounds, phosphonic acid compounds, formic acid compounds, boron hydride compounds and the like can be employed. In particular, however, palladium film obtained from an electroless palladium plating bath containing phosphinic acid or a phosphinate salt such as sodium phosphinate have better solder joint properties and wire bonding properties than palladium film obtained from an electroless palladium plating bath making use of a reducing agent other than that mentioned above.
Since a long time ago, there have been proposed numerous electroless palladium plating baths with phosphinate compounds contained as reducing agents (see Japanese Patent Publication No. Sho 46-26764 (Patent Document 1)). These plating baths are, however, accompanied by a problem in that they are poor in bath stability and are decomposed in short time. It has, therefore, been proposed to incorporate an additive with a view to improving the bath stability (Japanese Patent Laid-open No. Sho 62-124280 (Patent Document 2), Japanese Patent Publication No. Sho 53-37045 (Patent Document 3), Japanese Patent Laid-open No. Hei 5-39580 (Patent Document 4), Japanese Patent Laid-open No. Hei 8-269727 (Patent Document 5)). However, these proposals have not led to improvements in storage stability and the bath stability during continued use to such extent as making the baths withstandable their use on an industrial scale in the field of electronic industry or the like.
Patent Document 1:                Japanese Patent Publication No. Sho 46-26764        
Patent Document 2:                Japanese Patent Laid-open No. Sho 62-124280        
Patent Document 3:                Japanese Patent Publication No. Sho 53-37045        
Patent Document 4:                Japanese Patent Laid-open No. Hei 5-39580        
Patent Document 5:                Japanese Patent Laid-open No. Hei 8-269727        